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here goes nothing...

Well, I just bought a rear axle subassembly from an E39 on eBay. I'm going to check if we can swap some of the M5 aluminum rear suspension parts for the steel ones of the E38. I have done some research already and it seems that the E39 geometry is the same as the E38 except that the track is narrower (narrower subframe). Here is what I know so far (all in reference to the rear axle):

1) E38 brakes work on an E39 if you use the E38 parking brake pads.
2) forward upper control arm is the same on E38 and E39
3) Integral link is the same on E38 and E39
4) I have been told the swingarm can be swapped, but you need different size bushings and you need to solder on some tabs to the subframe so that the excentric bolt will function properly.

That just leaves the rear upper control arm and shock mount to verify.

The shock mount on the spindle is in about the right place (just from looking at pictures) and I know from the factory frame dimension diagrams for the Z8 and the E39 that the shock mounts to the frame in exactly the same place on each car. There may be a spacer required for the spindle mount, but the geometry seems identical between the two cars.

Finally, if as surmised above, all the other bits swap, the rear upper control arm should as well, apart from maybe adjusting the bushing size.

Nothing is a done deal, but I'm cautiously optimistic this might just work. What I am wondering, though, is why after 10 years has no one tried this? Or have they? It seems too good to be true. I'll know one way or another hopefully this weekend.

Also: swapping the subframes (again aluminum for steel) would take a significant amount of surgery to increase the width and I'm not sure about the differential mounting points being the same, so that's out of the picture (at least for now).

Nothing is a done deal, but I'm cautiously optimistic this might just work. What I am wondering, though, is why after 10 years has no one tried this? Or have they? It seems too good to be true. I'll know one way or another hopefully this weekend.

This sounds like an great project, and I'm excited to hear how you progress.

To answer your question, I think it's just a lack of demand. Short of the PP, wheels/tire, and maybe exhaust, most owners are very content to keep the car as is. Like you, I like the idea of less weight in the car, but for most, it's just not a priority. Now add to that a low production population and the potential for collecting and you have a very small group that want to max out the car for performance, etc. Those that desire more out of the car, probably just buy something else to go along side.

My guess is Fisker wanted the wider track of the E38...and they didn't mind the weight so much because it's in the back and helped them achieve the 50/50 weight distribution. Shame they didn't use a transaxle to get the 50/50 distribution...instead of sticking lead weights in the back.

The simplest answer seems the most likely: as a product, it does just fine with the off the shelf mix of e38, e39, and e65 parts, especially considering its lower weight than all of its doner vehicles. There are lots of areas where the Z8 could be better, but like any big project/product inevitable trade offs happen.

Also, I seem to recall that other than the assembly of the press cars, the Z8 has no affiliation with M GMBH. I also seem to recall that the S62 wasn't selected for the car until late in the project, with a modified version of the standard M62 (similar to the Alpina and X5is) being the originally planned engine. So, including suspension components from the M5 would seem to be a non-fit for the project as chartered.

Would this suspension setup be better? Most likely. Will I follow in ZMates footsteps if successful? Probably, assuming it complements or improves the EU Alpina setup. Will it improve my enjoyment of the car for how I use it? Probably not, other than having something new to discuss at the next Z8 gathering.

It'd be nice to have some lighter axels from the BMW parts bin. Dreaming of a full Cartridge setup.

I have never taken the Z8 to the track. I would like to one day, but that requires a trip to Germany. Switzerland by federal law limits engine capacity to 250cc (four strokes) on race tracks. However, I have to say in general if I want to go racing, the tool I'm going to use is my Rotax DD2 kart. It's vastly more capable, fun, safe, and inexpensive in the case of a mistake.

For me cars are mobile sculpture and the engineering is just as beautiful as the form. So my objective is not to achieve the lowest lap time, but to get the engineering up to a point where it is worthy of the form...not that it was too shabby to begin with, but to me a car this beautiful deserves the best engineering as well.

Even if you're not a racer and don't care much about engineering, losing unsprung weight improves the ride significantly, especially if you can dial down the damping on the suspension. And if you ever accidently push things too hard around a bumpy corner, the improvement in grip might just make the difference between losing it and continuing on home.

Also, I seem to recall that other than the assembly of the press cars, the Z8 has no affiliation with M GMBH. ... So, including suspension components from the M5 would seem to be a non-fit for the project as chartered.

From what I can tell, the E39 M5 suspension is the same same as other E39 sedans apart from using the beefier cast swingarm of the E39 touring (station wagon) suspension. The 540 E39 also uses the cast swingarm.

Well, the E39 rear subassembly arrived. Initial measurements are promising, but nothing is a straight swap. Everything takes some modification. Relatively easy modification, though. Starting with the upper rearward control arm. You need to cut 17mm off one side of the bushing. The weight saving is relatively modest:500g per side (340g unsprung). The original Z8 control arm is 1300g, so percentage-wise this is a big improvement.

The swingarm seems to swap, but you need to add some tabs for the excentric bolt and you need to use the anti-roll bar link from the E39. The tabs are not structural and can be attached to the swingarm with a simple spot weld. I will post pictures later. It's an easy mod.

Finally, the spindle: haven't finished measuring it, but one thing is sure: the shock mount is 23mm too short and will require machining up a spacer. Again, though, this is a pretty easy mod.

OK, I finally finally had a weekend free to make some progress on this. So after buying out all the BMW rear suspension parts on ebay Germany, I finally have a solution.

The problem with swapping the aluminum rear spindle from the E39 with the Z8 steel part as originally planned is that the mounting point for the shock is 23mm too far outboard. It turns out, though, that the mounting point on the E65 rear spindle (also aluminum) is 5mm further inward than on the Z8. This is actually a good thing as it creates a few mm more space for bigger tires/wheels. However, it's unfortunately not a straight swap with the Z8 part because of the parking brakes and the mounting points for the brake calipers. Mounting the brake calipers is an easy fix, you just need a 10mm spacer. The parking brakes take a bit more work:

1) the emergency brake pads on the E65 are larger than the ones on the Z8, so you need to do some adjustments to use the Z8 brake pads on the E65 spindle.2) the mounting point for the emergency brake cable is 30mm farther away from brake pads on the E65 than on the Z8, so you need to either have a custom cable made up or do some more adjustments.

First the parts you need. In the first parts diagram showing the spindle:

E65 parts1 "spindle"14 "front control arm"19 "rear control arm"

E60 M5 parts4 "integral link" (you can use the Z8 part here, but the E60 M5 part is cast aluminum instead of pressed steel and 100 grams lighter). You have to find used ones, though, because BMW has reverted to pressed steel

In the second parts diagram showing the parking brakes:

E39 parts3 "support"

E65 parts2 "M10 bolts"

Z8rest

In the third parts diagram showing the brake disk:

E65 parts4 "supporting ring"

the brake dust shield won't fit the Brembo brakes, so I'm not bothering with it.

Finally, the bearing and flange (1 and 2) shown in the last parts diagram should be from an E39.

Now the adjustments:

1) 10mm spacer for mounting the brake caliper as discussed above. 2) Machine down the face where the ?support? mounts to the spindle by 5mm, you need the extra space so that the Z8 parts clear.3) Grind down the ?support? so it fits inside the Z8 brake disc4) Grind down the circumference of the ?supporting ring? by 3mm so it fits inside the Z8 brake disc.5) Enlarge the holes of the E39 ?support? so that an M10 bolt from the E65 fits.6) Machine off 18mm from where the emergency brake cable is inserted into the back of the spindle.

As mentioned earlier, the E65 mounting point for the brake cable is 30mm farther away from the brake pads than on the Z8, so machining the mounting point down by 18mm reduces the difference to 12mm. The remaining difference (12mm) I think lies within the adjustment range of the cable. If not, the bite point on the handbrake lever will just be a bit earlier than with the stock brake setup.

Unsprung weight saving is about 2.5kg per wheel: about a 30% reduction relative to the Z8 parts.

I?m still working on the swap of the E39 swingarms. This is a bit more involved because the eccentric bolt for wheel alignment is on the integral link on the Z8 and on the subframe on the E39. It can be done, but requires welding some tabs onto the Z8 subframe. I'm hoping for at least a half kilo per wheel of unsprung weight savings here--perhaps a kilo?

In the pictures, those are Z8 emergency brake pads mounted on the E65 spindle. I haven't ground down the "support", so you can see how it sticks out too far beyond the brake pads.

I am having some parking brake anchors machined up out of titanium to replace the steel units from the E65. The E65 anchors could be ground down, but I thought I'd take the opportunity to save a little more weight...around 120grams per side.

So the question: does anyone want me to make up a pair for them? I can also do the machining required on the aluminum spindles (as outlined above) for you while I'm getting mine done. This would make the swap a plug and play affair.

In addition, I am machining up some tabs to weld to the rear subframe to accommodate the eccentric bolt necessary to install the E39 aluminum swingarm. Installation of the tabs requires a little grinding and some welding.

No idea yet about costs for the machining, but if there is any interest I can get quotes based upon the number of sets indicated.

I'm always open and curious, but what kinds of costs are we talking about?

Hi Andrew, I was quoted 800EUR for two parking brake anchors. 4 would hopefully be a little less. The suspension bits are around 1500EUR. Then you need to add a little for the machining of the spindle. Still interested?